Ip

C H A PT ER 30

Internet Protocols

Background
The Internet protocols are the world’s most popular open-system (nonproprietary) protocol suite
because they can be used to communicate across any set of interconnected networks and are equally
well suited for LAN and WAN communications. The Internet protocols consist of a suite of
communication protocols, of which the two best known are the Transmission Control Protocol
(TCP) and the Internet Protocol (IP). The Internet protocol suite not only includes lower-layer
protocols (such as TCP and IP), but it also specifies common applications such as electronic mail,
terminal emulation, and file transfer. This chapter provides a broad introduction to specifications that
comprise the Internet protocols. Discussions include IP addressing and key upper-layer protocols
used in the Internet. Specific routing protocols are addressed individually in Part 6, Routing
Protocols.
Internet protocols were first developed in the mid-1970s, when the Defense Advanced Research
Projects Agency (DARPA) became interested in establishing a packet-switched network that would
facilitate communication between dissimilar computer systems at research institutions. With the
goal of heterogeneous connectivity in mind, DARPA funded research by Stanford University and
Bolt, Beranek, and Newman (BBN). The result of this development effort was the Internet protocol
suite, completed in the late 1970s.
TCP/IP later was included with Berkeley Software Distribution (BSD) UNIX and has since become
the foundation on which the Internet and the World Wide Web (WWW) are based.
Documentation of the Internet protocols (including new or revised protocols) and policies are
specified in technical reports called Request For Comments (RFCs), which are published and then
reviewed and analyzed by the Internet community. Protocol refinements are published in the new
RFCs. To illustrate the scope of the Internet protocols, Figure 30-1 maps many of the protocols of
the Internet protocol suite and their corresponding OSI layers. This chapter addresses the basic
elements and operations of these and other key Internet protocols.

Internet Protocols 30-1

Internet Protocol (IP)

Figure 30-1 Internet protocols span the complete range of OSI model layers.

OSI
Reference Model Internet Protocol Suite

Application NFS

FTP, Telnet,
Presentation XDR
SMTP, SNMP

Session RPC

Transport TCP, UDP

Network Routing Protocols IP ICMP

ARP, RARP

Link

Not Specified

Physical

ith2801

The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information
and some control information that enables packets to be routed. IP is documented in RFC 791 and
is the primary network-layer protocol in the Internet protocol suite. Along with the Transmission
Control Protocol (TCP), IP represents the heart of the Internet protocols. IP has two primary
responsibilities: providing connectionless, best-effort delivery of datagrams through an
internetwork; and providing fragmentation and reassembly of datagrams to support data links with
different maximum-transmission unit (MTU) sizes.

IP Packet Format
An IP packet contains several types of information, as illustrated in Figure 30-2.

30-2 Internetworking Technology Overview, June 1999

IP Packet Format

Figure 30-2 Fourteen fields comprise an IP packet.

32 bits

Version IHL Type-of-service Total length

Identification Flags Fragment offset

Time-to-live Protocol Header checksum

Source address

Destination address

Options (+ padding)

Data (variable)

S2539
The following discussion describes the IP packet fields illustrated in Figure 30-2:
• Version—Indicates the version of IP currently used.
• IP Header Length (IHL)—Indicates the datagram header length in 32-bit words.
• Type-of-Service—Specifies how an upper-layer protocol would like a current datagram to be
handled, and assigns datagrams various levels of importance.
• Total Length—Specifies the length, in bytes, of the entire IP packet, including the data and
header.
• Identification—Contains an integer that identifies the current datagram. This field is used to help
piece together datagram fragments.
• Flags—Consists of a 3-bit field of which the two low-order (least-significant) bits control
fragmentation. The low-order bit specifies whether the packet can be fragmented. The middle bit
specifies whether the packet is the last fragment in a series of fragmented packets. The third or
high-order bit is not used.
• Fragment Offset—Indicates the position of the fragment’s data relative to the beginning of the
data in the original datagram, which allows the destination IP process to properly reconstruct the
original datagram.
• Time-to-Live—Maintains a counter that gradually decrements down to zero, at which point the
datagram is discarded. This keeps packets from looping endlessly.
• Protocol—Indicates which upper-layer protocol receives incoming packets after IP processing is
complete.
• Header Checksum—Helps ensure IP header integrity.
• Source Address—Specifies the sending node.
• Destination Address—Specifies the receiving node.



• Options—Allows IP to support various options, such as security.
• Data—Contains upper-layer information.

IP Addressing
As with any other network-layer protocol, the IP addressing scheme is integral to the process of
routing IP datagrams through an internetwork. Each IP address has specific components and follows
a basic format. These IP addresses can be subdivided and used to create addresses for subnetworks,
as discussed in more detail later in this chapter.
Each host on a TCP/IP network is assigned a unique 32-bit logical address that is divided into two
main parts: the network number and the host number. The network number identifies a network and
must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part
of the Internet. An Internet Service Provider (ISP) can obtain blocks of network addresses from the
InterNIC and can itself assign address space as necessary. The host number identifies a host on a
network and is assigned by the local network administrator.

IP Address Format
The 32-bit IP address is grouped eight bits at a time, separated by dots, and represented in decimal
format (known as dotted decimal notation). Each bit in the octet has a binary weight (128, 64, 32,
16, 8, 4, 2, 1). The minimum value for an octet is 0, and the maximum value for an octet is 255.
Figure 30-3 illustrates the basic format of an IP address.

Figure 30-3 An IP address consists of 32 bits, grouped into four octets.

32 Bits

Network Host

8 Bits 8 Bits 8 Bits 8 Bits

Dotted
Decimal
Notation
172 • 16 • 122 • 204

IP Address Classes
IP addressing supports five different address classes: A, B,C, D, and E. Only classes A, B, and C are
available for commercial use. The left-most (high-order) bits indicate the network class. Table 30-1
provides reference information about the five IP address classes.


IP Address Classes

Table 30-1 Reference Information About the Five IP Address Classes

IP
Addre High-Or
ss der No. Bits
Class Format Purpose Bit(s) Address Range Network/Host Max. Hosts
A N.H.H.H1 Few large 0 1.0.0.0 to 126.0.0.0 7/24 16,777, 2142
organizations (224 – 2)
B N.N.H.H Medium-size 1, 0 128.1.0.0 to 14/16 65, 543 (216 –
organizations 191.254.0.0 2)
C N.N.N.H Relatively small 1, 1, 0 192.0.1.0 to 22/8 245 (28 – 2)
organizations 223.255.254.0
D N/A Multicast groups 1, 1, 1, 0 224.0.0.0 to N/A (not for N/A
(RFC 1112) 239.255.255.255 commercial use)
E N/A Experimental 1, 1, 1, 1 240.0.0.0 to N/A N/A
254.255.255.255
1 N = Network number, H = Host number.
2 One address is reserved for the broadcast address, and one address is reserved for the network.

Figure 30-4 illustrates the format of the commercial IP address classes. (Note the high-order bits in
each class.)

Figure 30-4 IP address formats A, B, and C are available for commercial use.

No. Bits 7 24

Class A 0 Network Host Host Host

128 64 32 16 8 4 2 1

14 16

Class B 1 0 Network Network Host Host

21 8

Class C Network Network Host
24143

1 1 0 Network

The class of address can be determined easily by examining the first octet of the address and
mapping that value to a class range in the following table. In an IP address of 172.31.1.2, for
example, the first octet is 172. Because 172 falls between 128 and 191, 172.31.1.2 is a Class B
address. Figure 30-5 summarizes the range of possible values for the first octet of each address class.



Figure 30-5 A range of possible values exists for the first octet of each address class.

Address First Octet High-Order
Class in Decimal Bits

Class A 1 Ð 126 0

Class B 128 Ð 191 10

Class C 192 Ð 223 110

Class D 224 Ð 239 1110

Class E 240 Ð 254 1111

24144
IP Subnet Addressing
IP networks can be divided into smaller networks called subnetworks (or subnets). Subnetting
provides the network administrator with several benefits, including extra flexibility, more efficient
use of network addresses, and the capability to contain broadcast traffic (a broadcast will not cross
a router).
Subnets are under local administration. As such, the outside world sees an organization as a single
network and has no detailed knowledge of the organization’s internal structure.
A given network address can be broken up into many subnetworks. For example, 172.16.1.0,
172.16.2.0, 172.16.3.0, and 172.16.4.0 are all subnets within network 171.16.0.0. (All 0s in the host
portion of an address specifies the entire network.)

IP Subnet Mask
A subnet address is created by “borrowing” bits from the host field and designating them as the
subnet field. The number of borrowed bits varies and is specified by the subnet mask. Figure 30-6
shows how bits are borrowed from the host address field to create the subnet address field.


IP Address Classes

Figure 30-6 Bits are borrowed from the host address field to create the subnet address
field.

Class B Address: Before Subnetting

1 0 Network Network Host Host

1 0 Network Network Subnet Host

Class B Address: After Subnetting

Subnet masks use the same format and representation technique as IP addresses. The subnet mask,
however, has binary 1s in all bits specifying the network and subnetwork fields, and binary 0s in all
bits specifying the host field. Figure 30-7 illustrates a sample subnet mask.

Figure 30-7 A sample subnet mask consists of all binary 1s and 0s.

Network Network Subnet Host

Binary
representation 11111111 11111111 11111111 00000000

24145
Dotted decimal
representation 255 255 255 0

Subnet mask bits should come from the high-order (left-most) bits of the host field, as Figure 30-8
illustrates. Details of Class B and C subnet mask types follow. Class A addresses are not discussed
in this chapter because they generally are subnetted on an 8-bit boundary.



Figure 30-8 Subnet mask bits come from the high-order bits of the host ﬁeld.

128 64 32 16 8 4 2 1

1 0 0 0 0 0 0 0 = 128

1 1 0 0 0 0 0 0 = 192

1 1 1 0 0 0 0 0 = 224

1 1 1 1 0 0 0 0 = 240

1 1 1 1 1 0 0 0 = 248

1 1 1 1 1 1 0 0 = 252

1 1 1 1 1 1 1 0 = 254

24146
1 1 1 1 1 1 1 1 = 255

Various types of subnet masks exist for Class B and C subnets.
The default subnet mask for a Class B address that has no subnetting is 255.255.0.0, while the subnet
mask for a Class B address 171.16.0.0 that speciﬁes eight bits of subnetting is 255.255.255.0. The
reason for this is that eight bits of subnetting or 28 – 2 (1 for the network address and 1 for the
broadcast address) = 254 subnets possible, with 28 – 2 = 254 hosts per subnet.
The subnet mask for a Class C address 192.168.2.0 that specifies five bits of subnetting is
255.255.255.248.With five bits available for subnetting, 25 – 2 = 30 subnets possible, with
23 – 2 = 6 hosts per subnet.
The reference charts shown in table 30–2 and table 30–3 can be used when planning Class B and C
networks to determine the required number of subnets and hosts, and the appropriate subnet mask.

Table 30-2 Class B Subnetting Reference Chart

Number of Bits Subnet Mask Number of Subnets Number of Hosts
2 255.255.192.0 2 16382
3 255.255.224.0 6 8190
4 255.255.240.0 14 4094
5 255.255.248.0 30 2046
6 255.255.252.0 62 1022
7 255.255.254.0 126 510
8 255.255.255.0 254 254
9 255.255.255.128 510 126
10 255.255.255.192 1022 62
11 255.255.255.224 2046 30
12 255.255.255.240 4094 14


IP Address Classes

13 255.255.255.248 8190 6
14 255.255.255.252 16382 2

Table 30-3 Class C Subnetting Reference Chart

2 255.255.255.192 2 62
3 255.255.255.224 6 30
4 255.255.255.240 14 14
5 255.255.255.248 30 6
6 255.255.255.252 62 2

How Subnet Masks are Used to Determine the Network Number
The router performs a set process to determine the network (or more speciﬁcally, the subnetwork)
address. First, the router extracts the IP destination address from the incoming packet and retrieves
the internal subnet mask. It then performs a logical AND operation to obtain the network number.
This causes the host portion of the IP destination address to be removed, while the destination
network number remains. The router then looks up the destination network number and matches it
with an outgoing interface. Finally, it forwards the frame to the destination IP address. Speciﬁcs
regarding the logical AND operation are discussed in the following section.

Logical AND Operation
Three basic rules govern logically “ANDing” two binary numbers. First, 1 “ANDed” with 1 yields
1. Second, 1 “ANDed” with 0 yields 0. Finally, 0 “ANDed” with 0 yields 0. The truth table provided
in table 30–4 illustrates the rules for logical AND operations.

Table 30-4 Rules for Logical AND Operations

Input Input Output
1 1 1
1 0 0
0 1 0
0 0 0

Two simple guidelines exist for remembering logical AND operations: Logically “ANDing” a 1 with
a 1 yields the original value, and logically “ANDing” a 0 with any number yields 0.
Figure 30-9 illustrates that when a logical AND of the destination IP address and the subnet mask is
performed, the subnetwork number remains, which the router uses to forward the packet.


Internet Routing

Figure 30-9 Applying a logical AND the destination IP address and the subnet mask
produces the subnetwork number.

Network Subnet Host

Destination IP
Address 171.16.1.2 10101011 00010000 00000001 00000010

Subnet 255.255.255.0 11111111 11111111 11111111 00000000
Mask
10101011 00010000 00000001 00000000

171 16 1 0

24147
Address Resolution Protocol (ARP) Overview
For two machines on a given network to communicate, they must know the other machine’s physical
(or MAC) addresses. By broadcasting Address Resolution Protocols (ARPs), a host can dynamically
discover the MAC-layer address corresponding to a particular IP network-layer address.
After receiving a MAC-layer address, IP devices create an ARP cache to store the recently acquired
IP-to-MAC address mapping, thus avoiding having to broadcast ARPS when they want to recontact
a device. If the device does not respond within a specified time frame, the cache entry is flushed.
In addition to the Reverse Address Resolution Protocol (RARP) is used to map MAC-layer addresses
to IP addresses. RARP, which is the logical inverse of ARP, might be used by diskless workstations
that do not know their IP addresses when they boot. RARP relies on the presence of a RARP server
with table entries of MAC-layer-to-IP address mappings.

Internet Routing
Internet routing devices traditionally have been called gateways. In today’s terminology, however,
the term gateway refers specifically to a device that performs application-layer protocol translation
between devices. Interior gateways refer to devices that perform these protocol functions between
machines or networks under the same administrative control or authority, such as a corporation’s
internal network. These are known as autonomous systems. Exterior gateways perform protocol
functions between independent networks.
Routers within the Internet are organized hierarchically. Routers used for information exchange
within autonomous systems are called interior routers, which use a variety of Interior Gateway
Protocols (IGPs) to accomplish this purpose. The Routing Information Protocol (RIP) is an example
of an IGP.
Routers that move information between autonomous systems are called exterior routers. These
routers use an exterior gateway protocol to exchange information between autonomous systems. The
Border Gateway Protocol (BGP) is an example of an exterior gateway protocol.

Note Specific routing protocols, including BGP and RIP, are addressed in individual chapters
presented in Part 6 later in this book.


IP Routing

IP Routing
IP routing protocols are dynamic. Dynamic routing calls for routes to be calculated automatically at
regular intervals by software in routing devices. This contrasts with static routing, where routers are
established by the network administrator and do not change until the network administrator changes
them.
An IP routing table, which consists of destination address/next hop pairs, is used to enable dynamic
routing. An entry in this table, for example, would be interpreted as follows: to get to network
172.31.0.0, send the packet out Ethernet interface 0 (E0).
IP routing specifies that IP datagrams travel through internetworks one hop at a time. The entire route
is not known at the onset of the journey, however. Instead, at each stop, the next destination is
calculated by matching the destination address within the datagram with an entry in the current
node’s routing table.
Each node’s involvement in the routing process is limited to forwarding packets based on internal
information. The nodes do not monitor whether the packets get to their final destination, nor does IP
provide for error reporting back to the source when routing anomalies occur. This task is left to
another Internet protocol, the Internet Control-Message Protocol (ICMP), which is discussed in the
following section.

Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is a network-layer Internet protocol that provides
message packets to report errors and other information regarding IP packet processing back to the
source. ICMP is documented in RFC 792.

ICMP Messages
ICMPs generate several kinds of useful messages, including Destination Unreachable, Echo Request
and Reply, Redirect, Time Exceeded, and Router Advertisement and Router Solicitation. If an ICMP
message cannot be delivered, no second one is generated. This is to avoid an endless flood of ICMP
messages.
When an ICMP destination-unreachable message is sent by a router, it means that the router is unable
to send the package to its final destination. The router then discards the original packet. Two reasons
exist for why a destination might be unreachable. Most commonly, the source host has specified a
nonexistent address. Less frequently, the router does not have a route to the destination.
Destination-unreachable messages include four basic types: network unreachable, host unreachable,
protocol unreachable, and port unreachable. Network-unreachable messages usually mean that a
failure has occurred in the routing or addressing of a packet. Host-unreachable messages usually
indicates delivery failure, such as a wrong subnet mask. Protocol-unreachable messages generally
mean that the destination does not support the upper-layer protocol specified in the packet.
Port-unreachable messages imply that the TCP socket or port is not available.
An ICMP echo-request message, which is generated by the ping command, is sent by any host to test
node reachability across an internetwork. The ICMP echo-reply message indicates that the node can
be successfully reached.
An ICMP Redirect message is sent by the router to the source host to stimulate more efficient
routing. The router still forwards the original packet to the destination. ICMP redirects allow host
routing tables to remain small because it is necessary to know the address of only one router, even if
that router does not provide the best path. Even after receiving an ICMP Redirect message, some
devices might continue using the less-efficient route.


Transmission Control Protocol (TCP)

An ICMP Time-exceeded message is sent by the router if an IP packet’s Time-to-Live field
(expressed in hops or seconds) reaches zero. The Time-to-Live field prevents packets from
continuously circulating the internetwork if the internetwork contains a routing loop. The router then
discards the original packet.

ICMP Router-Discovery Protocol (IDRP)
IDRP uses Router-Advertisement and Router-Solicitation messages to discover the addresses of
routers on directly attached subnets. Each router periodically multicasts Router-Advertisement
messages from each of its interfaces. Hosts then discover addresses of routers on directly attached
subnets by listening for these messages. Hosts can use Router-Solicitation messages to request
immediate advertisements rather than waiting for unsolicited messages.
IRDP offers several advantages over other methods of discovering addresses of neighboring routers.
Primarily, it does not require hosts to recognize routing protocols, nor does it require manual
configuration by an administrator.
Router-Advertisement messages enable hosts to discover the existence of neighboring routers, but
not which router is best to reach a particular destination. If a host uses a poor first-hop router to reach
a particular destination, it receives a Redirect message identifying a better choice.

The TCP provides reliable transmission of data in an IP environment. TCP corresponds to the
transport layer (Layer 4) of the OSI reference model. Among the services TCP provides are stream
data transfer, reliability, efficient flow control, full-duplex operation, and multiplexing.
With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence
numbers. This service benefits applications because they do not have to chop data into blocks before
handing it off to TCP. Instead, TCP groups bytes into segments and passes them to IP for delivery.
TCP offers reliability by providing connection-oriented, end-to-end reliable packet delivery through
an internetwork. It does this by sequencing bytes with a forwarding acknowledgment number that
indicates to the destination the next byte the source expects to receive. Bytes not acknowledged
within a specified time period are retransmitted. The reliability mechanism of TCP allows devices
to deal with lost, delayed, duplicate, or misread packets. A time-out mechanism allows devices to
detect lost packets and request retransmission.
TCP offers efficient flow control, which means that, when sending acknowledgments back to the
source, the receiving TCP process indicates the highest sequence number it can receive without
overflowing its internal buffers.
Full-duplex operation means that TCP processes can both send and receive at the same time.
Finally, TCP’s multiplexing means that numerous simultaneous upper-layer conversations can be
multiplexed over a single connection.

TCP Connection Establishment
To use reliable transport services, TCP hosts must establish a connection-oriented session with one
another. Connection establishment is performed by using a “three-way handshake” mechanism.
A three-way handshake synchronizes both ends of a connection by allowing both sides to agree upon
initial sequence numbers. This mechanism also guarantees that both sides are ready to transmit data
and know that the other side is ready to transmit as well. This is necessary so that packets are not
transmitted or retransmitted during session establishment or after session termination.


Positive Acknowledgment and Retransmission (PAR)

Each host randomly chooses a sequence number used to track bytes within the stream it is sending
and receiving. Then, the three-way handshake proceeds in the following manner:
The first host (Host A) initiates a connection by sending a packet with the initial sequence number
(X) and SYN bit set to indicate a connection request. The second host (Host B) receives the SYN,
records the sequence number X, and replies by acknowledging the SYN (with an ACK = X + 1).
Host B includes its own initial sequence number (SEQ = Y). An ACK = 20 means the host has
received bytes 0 through 19 and expects byte 20 next. This technique is called forward
acknowledgment. Host A then acknowledges all bytes Host B sent with a forward acknowledgment
indicating the next byte Host A expects to receive (ACK = Y + 1). Data transfer then can begin.

Positive Acknowledgment and Retransmission (PAR)
A simple transport protocol might implement a reliability-and-flow-control technique where the
source sends one packet, starts a timer, and waits for an acknowledgment before sending a new
packet. If the acknowledgment is not received before the timer expires, the source retransmits the
packet. Such a technique is called positive acknowledgment and retransmission (PAR).
By assigning each packet a sequence number, PAR enables hosts to track lost or duplicate packets
caused by network delays that result in premature retransmission. The sequence numbers are sent
back in the acknowledgments so that the acknowledgments can be tracked.
PAR is an inefficient use of bandwidth, however, because a host must wait for an acknowledgment
before sending a new packet, and only one packet can be sent at a time.

TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it
enables hosts to send multiple bytes or packets before waiting for an acknowledgment.
In TCP, the receiver specifies the current window size in every packet. Because TCP provides a
byte-stream connection, window sizes are expressed in bytes. This means that a window is the
number of data bytes that the sender is allowed to send before waiting for an acknowledgment. Initial
window sizes are indicated at connection setup, but might vary throughout the data transfer to
provide flow control. A window size of zero, for instance, means “Send no data.”
In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send
(numbered 1 to 10) to a receiver who has a window size of five. The sender then would place a
window around the first five bytes and transmit them together. It would then wait for an
acknowledgment.
The receiver would respond with an ACK = 6, indicating that it has received bytes 1 to 5 and is
expecting byte 6 next. In the same packet, the receiver would indicate that its window size is 5. The
sender then would move the sliding window five bytes to the right and transmit bytes 6 to 10. The
receiver would respond with an ACK = 11, indicating that it is expecting sequenced byte 11 next. In
this packet, the receiver might indicate that its window size is 0 (because, for example, its internal
buffers are full). At this point, the sender cannot send any more bytes until the receiver sends another
packet with a window size greater than 0.



TCP Packet Format
Figure 30-10 illustrates the fields and overall format of a TCP packet.

Figure 30-10 Twelve fields comprise a TCP packet.

Source port Destination port

Sequence number

Acknowledgment number

Data offset Reserved Flags Window

Checksum Urgent pointer

Options (+ padding)

Data (variable)

S1344a

TCP Packet Field Descriptions
The following descriptions summarize the TCP packet fields illustrated in Figure 30-10:
• Source Port and Destination Port—Identifies points at which upper-layer source and destination
processes receive TCP services.
• Sequence Number—Usually specifies the number assigned to the first byte of data in the current
message. In the connection-establishment phase, this field also can be used to identify an initial
sequence number to be used in an upcoming transmission.
• Acknowledgment Number—Contains the sequence number of the next byte of data the sender of
the packet expects to receive.
• Data Offset—Indicates the number of 32-bit words in the TCP header.
• Reserved—Remains reserved for future use.
• Flags—Carries a variety of control information, including the SYN and ACK bits used for
connection establishment, and the FIN bit used for connection termination.
• Window—Specifies the size of the sender’s receive window (that is, the buffer space available for
incoming data).
• Checksum—Indicates whether the header was damaged in transit.
• Urgent Pointer—Points to the first urgent data byte in the packet.
• Options—Specifies various TCP options.
• Data—Contains upper-layer information.


User Datagram Protocol (UDP)

User Datagram Protocol (UDP)
The User Datagram Protocol (UDP) is a connectionless transport-layer protocol (Layer 4) that
belongs to the Internet protocol family. UDP is basically an interface between IP and upper-layer
processes. UDP protocol ports distinguish multiple applications running on a single device from one
another.
Unlike the TCP, UDP adds no reliability, flow-control, or error-recovery functions to IP. Because of
UDP’s simplicity, UDP headers contain fewer bytes and consume less network overhead than TCP.
UDP is useful in situations where the reliability mechanisms of TCP are not necessary, such as in
cases where a higher-layer protocol might provide error and flow control.
UDP is the transport protocol for several well-known application-layer protocols, including Network
File System (NFS), Simple Network Management Protocol (SNMP), Domain Name System (DNS),
and Trivial File Transfer Protocol (TFTP).
The UDP packet format contains four fields, as shown in Figure 30-11. These include source and
destination ports, length, and checksum fields.

Figure 30-11 A UDP packet consists of four fields.

32 Bits

Source Port Destination Port

Length Checksum
24148

Source and destination ports contain the 16-bit UDP protocol port numbers used to demultiplex
datagrams for receiving application-layer processes. A length field specifies the length of the UDP
header and data. Checksum provides an (optional) integrity check on the UDP header and data.

Internet Protocols Application-Layer Protocols
The Internet protocol suite includes many application-layer protocols that represent a wide variety
of applications, including the following:
• File Transfer Protocol (FTP)—Moves files between devices
• Simple Network-Management Protocol (SNMP)—Primarily reports anomalous network
conditions and sets network threshold values
• Telnet—Serves as a terminal emulation protocol
• X Windows—Serves as a distributed windowing and graphics system used for communication
between X terminals and UNIX workstations
• Network File System (NFS), External Data Representation (XDR), and Remote Procedure Call
(RPC)—Work together to enable transparent access to remote network resources
• Simple Mail Transfer Protocol (SMTP)—Provides electronic mail services
• Domain Name System (DNS)—Translates the names of network nodes into network addresses
Table 30-5 lists these higher-layer protocols and the applications that they support.


Internet Protocols Application-Layer Protocols

Table 30-5 Higher-Layer Protocols and Their Applications

Application Protocols
File transfer FTP
Terminal emulation Telnet
Electronic mail SMTP
Network management SNMP
Distributed ﬁle services NFS, XDR, RPC, X Windows


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